1. Field of the Invention
The present invention relates generally to optical unit actuators for information recording/reproducing apparatus, and more particularly to an actuator for driving an optical unit including an objective lens for recording and reproducing information on an optical recording medium such as an optical disk.
2. Description of the Prior Art
With the spread use of optical disk drive units as an external storage for computers, the demand for a compact and high-speed unit has increased.
More specifically, there have been a demand for high speed rotation of an optical disk for the purpose of realizing a high data transfer rate, and a demand for a compact and flat structure resulting from the tendency toward an optical disk of a smaller diameter.
The speeding-up and compacting of the optical disk unit involves several problems to be solved. A main problem is the maintenance of control characteristics of an optical disk, an optical head and a control circuit for the optical head.
An optical unit actuator of a conventional information recording/reproducing apparatus will be described below with reference to FIGS. 6 and 7 of the accompanying drawings, in which FIG. 6 is an exploded perspective view of a main portion of the optical unit actuator, and FIG. 7 is a cross-sectional view taken along line VII--VII of FIG. 6.
As shown in FIG. 6, the actuator includes an objective lens holder 1 to which an objective lens 2, two opposed tracking coils 3 and a focusing coil 4 are secured by bonding. The objective lens holder 1 has a pair of aligned central cylindrical projections (not designated) which are fitted in the central holes of two opposed plate springs 6 and bonded to the plate springs 6. The outer peripheral portions of the respective plate springs 6 are secured by bonding to a carriage 5, so that the objective lens holder 1 is mounted in the carriage 5. The objective lens holder 1 is displaceable relative to an optical disk 7 both in a focusing direction and in a tracking direction. In order to move the carriage 5 in the radial direction of the optical disk 7, two opposed linear motor coils 8, 9 are wound around bobbins 10, 11 which are secured by bonding to opposite sides of the carriage 5. A fixed optical base 12 generates a light beam used for recording/reproducing information on the optical disk 7. The light beam is projected on a mirror 13 fixed on the bobbin 11 and then is reflected by this mirror 13 toward the objective lens 2. A magnetic circuit which constitutes a fixed unit or member as against the displacement of the objective lens 2 is composed of two opposed back yokes 14, 14, magnets 15, 15 secured by bonding to the respective back yokes 14, 14, two confronting yokes 16, 16 facing the corresponding magnets 15, 15 and two opposed side yokes 17, 18 for completing a closed magnetic circuit. The components of the magnetic circuit are assembled together by a rigid connection in terms of vibration. The carriage 5 is assembled with the magnetic circuit such that parts of the tracking coils 3, focusing coil 4 and linear motor coils 8, 9 are disposed in magnetic gas defined between the magnets 15 and the confronting yokes 16. The magnetic circuit is common to the tracking coils 3, focusing coil 4, and linear motor coils 8, 9. A pair of shafts 19 extends between the side yokes 17, 18 and resiliently retained on the side yokes 17, 18 by means of plate springs 20. The shafts 19 are slidably received in a pair of plain bearings 5a, 5a, respectively, of the carriage 5 so that the carriage 5 is slidable along the shaft 19 in the radial direction of the optical disk 7. The fixed optical base 12 includes a semiconductor laser for generating a light beam for recording and reproducing information on the optical disk 7, optical components, a photoelectric transducer, etc. and is secured to the side yoke 17. The optical disk 7 is rotated at a predetermined speed by a spindle motor 22 secured to the side yoke 18.
The conventional optical unit actuator of the foregoing construction involves problems in maintaining the control characteristics of the optical disk, optical head and control circuit for the optical head when an attempt is made to speed up the rotation of the optical disk for the purpose of increasing the data transfer rate and to minimize the overall size of the unit to conform to the tendency toward a small-diameter optical disk.
The problems associated with the conventional optical unit actuator will be described below with reference to FIG. 8. FIG. 8 shows amplitude-to-vibration performance curves of various components of the actuator obtained when the focusing coil 4 is excited to move the objective lens holder 1 in the focusing direction of the optical disk 7. In this Figure, the solid line indicated by a shows the vibrational characteristic of the objective lens holder 1, and the chain line indicated by b.c shows the vibrational characteristic of the magnets 15 and the back yokes 14.
To insure a reliable recording/reproducing of information on an optical disk 7, it is desired that the objective lens holder 1 does not induce parasitic oscillation even at a high frequency, as indicated by the solid line a, but generates an acceleration which is capable of following a dynamic radial runout and an axial deflection of the optical disk 7.
The acceleration of the objective lens holder 1 means that the magnet 15 and the back yoke 14 are caused to vibrate, as indicated by the chain line b.c, by a reaction force resulting from acceleration of the objective lens holder 1. In this instance, the magnet 15 and the corresponding back yoke 14 are integral with each other and hence vibrate at the same amplitude of vibration. The difference in amplitude between the objective lens holder 1 and the magnet 15 or between the objective lens holder 1 and the back yoke 14 is determined by the ratio of a mass of the objective lens holder 1 to a mass of the magnetic circuit which constitutes a fixed unit or member as against the displacement of the movable objective lens holder 1.
The vibrational energy which vibrates the magnet 15 and the back yoke 14 is transmitted to the fixed optical base 12 via the side yoke 17. Since the side yoke 17 is joined with the magnet 15 and the back yoke 14 in rigid connection in terms of vibration, the fixed optical base 12 is vibrated.
When optical disk 7 is rotating at a high speed in the range of 2400-3600 rpm so as to provide a high data transfer rate, a great acceleration is produced due to the dynamic radial runout and the axial deflection of the rotating optical disk 7. To enable the objective lens 2 to accurately follow the thus rotating optical disk 7 during the recording/reproducing operation, the objective lens holder 1 is highly accelerated. A very large acceleration thus produced creates a considerably large vibrational energy which is in turn transmitted to the fixed optical base 12.
The fixed optical base 12 is thus vibrating under the influence of the vibrational energy. During that time, various optical components, a photoelectric transducer, etc. which are disposed in an optical path extending between the fixed optical base 12 and the optical disk 7 are vibrated. As a result, a control system used for controlling the objective lens 2 to follow the optical disk 7 is in the oscillated condition. Under such condition, the objective lens 2 is no longer possible to accurately follow the movement of a recording surface of the optical disk 7. Thus, an accurate recording/reproducing operation is difficult to achieve.
An optical information recording/reproducing apparatus for use with a small-diameter optical disk must be compact in size and low in profile. Consequently, the optical disk per se is as thin as possible. To this end, an optical disk of a single plate structure is used. The optical disk of the single plate structure is low in rigidity and hence is likely to induce resonance vibration when subjected to a reaction force resulting from the acceleration of the objective lens holder 1 which is transmitted via the spindle motor 22. When the optical disk 7 undergoes resonance vibration, the control system used for controlling the objective lens 2 to follow the recording surface of the optical disk 7 is caused to oscillate. Thus, an accurate recording/reproducing of information on the optical disk is no longer possible.